Recently, studies about robots have been prosperous. Developing the sensing functions of artificial intelligence, such as the tactile, thermal, visual, and auditory sensors allows the robots to effectively and safely interact with human beings or the external environment. Specifically, the tactile sensors are an indispensable interface for the robots to contact the external environment. The tactile sensors can be primarily categorized into piezoresistive, capacitive, piezoelectric, and optical types of tactile sensors. In general applications, piezoresistive and capacitive tactile sensors are two common types of tactile sensors.
Most of the tactile sensors used in the robots sense normal forces (contact pressure). However, when a robotic claw grabs an object, the strength of the shear force (e.g. frictional force) may need to be sensed as well as the strength of the grabbing force, so as to predict the occurrence of a sliding movement, and feedback and adjust the grabbing force to prevent the object from falling. Therefore, the tactile sensors not only need to sense a normal force, namely the grabbing force, but also to sense a shear force perpendicular to the grabbing force in order to dynamically obtain the information about the frictional force and the sliding movements experienced by the object. In addition, in the application of assembly of parts in the production line, such as connectors, plugs and sockets, the tactile sensors adapted for sensing the shear force may prevent the parts from being damaged due to inappropriate collision during assembling.